PROJECT ABSTRACT Osteoarthritis (OA) is the most common form of arthritis. Patients with OA lack effective medical treatments to regenerate damaged cartilage and there is a need for developing novel therapies to treat joint disorders. Joint homeostasis is maintained preserving the articular cells to proceed down the differentiation pathway that leads to chondrocyte hypertrophy and bone replacement, such it occurs in the growth plate cartilage. If OA is the result of the failure of progenitor cells to establish a corret microenvironment, exploring how joints form can lead us to understand how cartilage degenerates and develop drugs that are able to reactivate the forming mechanism. Our long-term goal is to elucidate how joint progenitor cells express generative and regenerative joint genes in order to manipulate their expression for therapeutic purpose in OA. The overall objective is to determine whether the regulation of the cytokine monocyte chemoattractant protein-5 (MCP-5) is a key down- stream mediator of TGF-alpha (a) type II receptor (TaRII) signaling in joint maintenance and post-traumatic OA. Our central hypothesis is that a controlled expression of MCP-5 is needed in the interzone to allow proper joint formation during development and is also crucial in adulthood to maintain joint integrity and to prevent OA degeneration. The rationale for the proposed research is that a cytokine controlled environment is a common mechanism in joint to promote development and maintain homeostasis. Based on our preliminary data we are proposing two Specific Aims: 1) To determine whether TaRII signaling is needed to maintain joint integrity through down-regulation of MCP-5 expression; 2) To determine the role of MCP-5/TaRII axis in post-traumatic OA. In the first aim, we will generate the Prx1CreERT2/Tgfbr2lox/lox mouse to obtain temporal and limb-specific control of TaRII expression using a tamoxifen-inducible-Cre transgenic in which we will evaluate OA development in the presence or absence of MCP-5 signaling (by chemical and receptor manipulation). In the second aim, we will: A) induce post-traumatic OA by performing a destabilization of medial meniscus in Tgfbr2-a-Gal-GFP-BAC mice (imaging reporters of TaRII expression) and analyze expression of MCP-5 in concomitance with TaRII during OA development; B) induce mechanical loading on isolated TaRII expressing cells and evaluate whether cell deformation leads to progressive loss of responsiveness to TGF-a with consequent increase in MCP-5 expression. This proposal is innovative because provides a new perspective to evaluate the OA process: OA is not only seen as the result of the damage induced by a systemic influx of cytokines against a passive target (the joint), but rather the failure of an actve cell joint population to maintain a controlled cytokine environment. Our studies will provide critical insights on joint development homeostasis and OA pathophysiology.